There is an increasing demand on mobile wireless operators to provide voice and high-speed data services, and at the same time, mobile network operators wish to support more users per basestation to reduce overall network costs and make services more affordable to subscribers. As a result, wireless systems that enable higher data rates and higher capacities to the user equipment are needed. The available spectrum for wireless services is limited, however, and the prior attempts to increase traffic within a fixed bandwidth have increased interference in the system and degraded signal quality.
Various schemes have been implemented on orthogonal frequency domain multiple access (OFDMA) systems to increase system performance. Technologies like multiple input multiple output (MIMO), orthogonal frequency division multiplexing (OFDM), and advanced error control codes enhance per-link throughput, but these technologies do not solve all problems encountered in the communication network.
Wireless communications networks are typically divided into cells, with each of the cells further divided into cell sectors. A base station transceiver unit is provided in each cell to enable wireless communications with mobile stations located within the cell site location. Reference signals are transmitted from cell site base station transceivers on the cell site where the user equipment is located (eNodeB or serving cell sites), as well as being transmitted from base station transceivers on various neighbor cell sites (neighbor cell sites) located around the serving cell site.
Reference signals are used by user equipment on an Orthogonal Frequency-Division Multiple Access (OFDMA) system, such as a 3GPP and LTE mobile wireless communication systems, to assist in establishing the location of user equipment on the mobile wireless communication system. In one form of location analysis, the user equipment uses the reference signals received from the serving and neighboring cell sites to determine the user equipment location to determine a time difference of arrival between reference signals transmitted from the serving cell site and the neighbor cell sites. By calculating a time difference of arrival for the reference signals, the user equipment or other components on the network can perform a triangulation calculation to accurately determine the location of the user equipment on the network. That location information is used to adjust the power of transmission signals to and from the user equipment so as to reduce interference with other signals on the network and improve the overall accuracy of the signal transmissions to and from the user equipment.
Neighbor cell hearability is the ability of the user equipment to detect, or “hear,” reference signals from neighbor cell sites. Reference signals from the serving cell sites and neighboring cell sites, however, must be accurately detected, or “heard,” by user equipment in order to be used in the location analysis. One problem encountered in neighbor cell hearability arises when user equipment is located near to the center of the serving cell site such that reference signals from neighbor cell sites are too weak for proper detection by the user equipment. In this situation, the reference signals from neighbor cell sites is too weak for the user equipment to accurately estimate the time difference of arrivals between the reference signal from the serving and various neighbor cell sites, which hinders the triangulation location analysis conducted by the user equipment.
Known prior art systems and proposals do not adequately address the neighbor cell hearability problem that arises when user equipment is located near to the center of the serving cell site, and these known systems and proposals include the following: (1) 3GPP TS 36.133 v8.4.0, “E-UTRA Requirements for support of radio resource management,” (2) 3GPP TS 36.214 v8.5.0, ‘E-UTRA; Physical layer measurements’, December 2008, (3) 3GPP TS 36211 v8.5.0, ‘E-UTRA: Physical channels and modulation’, December 2008, (4) RI-090053, ‘Improving the hearability of LTE Positioning Service’, Alcatel-Lucent, RAN1.55bis, Ljubljana, Slovenia, January 2009, [1] (5) R1-090321, ‘Positioning Support for LTE Rel-9—RANI Specific Issues’, Motorola, RAN155bis, Ljubljana, Slovenia, January 2009, and (6) R1-090353 ‘On OTDOA in LTE’, Qualcomm Europe, RAN1-55bis, Ljubljana, Slovenia, January 2009 [3].
In reference (4) and (6) identified above, different additional reference signal patterns are proposed, but both these proposals do not provide a workable or improved solution for the neighbor cell hearability problem when user equipment is located near the serving cell site.
In reference proposal (4) identified above, one resource block (RB) for transmitting a new reference signal RS pattern, called the LCS-RS, must be scheduled. However, the joint scheduling of the resource block RB for transmitting the reference signal (LCS-RS) requires coordination between various neighbor cell sites which is not currently supported by network communication systems. Further, reference proposal (4) above requires the cell sites to be synchronous, the new reference signal LCS-RS pattern has a different structure as compared to the cell-specific reference signal RS, called the CRS that is defined in the current specification. Lastly, collisions between clusters of neighbor cells may still arise unless coordination is done extensively over a larger cluster of the network. In order to implement this proposal identified by reference (4) above, a new type of reference signal is required that is not recognized by the current network systems and large scale synchronous coordination of system components would need to be coordinated. This proposal, therefore, requires changes to the existing system that are too extensive to be workable or practical.
With respect to the reference proposal (6) identified above, the proposed reference signal (E-IRDL RS) follows a very different structure as compared to that of the cell-specific reference signal (CRS) in the existing standard, which requires the introduction of new, and complex, technology in the receivers. In order to implement this proposal identified by reference (6) above, a new type of reference signal is required that is not recognized by the current network systems and the implementation of new technology in receivers would be required. This proposal, therefore, also requires changes to the existing system that are too extensive to be workable or practical.
Simulations were also conducted on the reference proposals (4), (5) and (6) identified above in a multi-cell, multi-sector deployment scenario, with user equipment simulated to be located with uniform randomness in the serving cell site. 3GPP simulation results for Cases 1 and 3 are shown below with an MD intra-frequency measurement sensitivity requirement is set to be SGH_RP˜126 dBm as defined in 3GPP TS 36.133 v8.4.0, “E-UTRA Requirements for support of radio resource management.”
In the simulations, reuse mechanisms could be achieved in frequency, time, and/or code domain, but no specific reuse mechanism has been assumed. The simulations did, however, assume reuse factors of 1, 3 and 6. The Gil distributions of best N neighbor cell signals as observed by each UE are captured and plotted as shown in FIGS. 1-3. The geometry (Gil) distribution of the signal from the serving cell is also plotted for comparison. The cell hearability requirement defined in 3GPP TS 36.133 v8.4.0, “E-UTRA Requirements for support of radio resource management” is SCH E=˜6 dB. In the present simulation study, hearability requirements are assumed to be −6, −8 or −10 dB. The distribution of the number of neighbor cells with detectable signal are plotted as shown in FIGS. 4-6.
From the simulation data taken, the applicants observed the following:                For a Reuse Factor of 1, the probability that a UE can detect 3 or more sites is less than 20%, even when ISO=500 m (Case 1) the hearability CII threshold is as low as −10 dB;        For a Reuse Factor of 3:                    In Case 1, UE can detect 3 or more sites with probability of about 69% when CII threshold is −6 dB, 77% at −8 dB, and 85% at −10 dB;            In Case 3, LIE can detect 3 or more sites with probability of about 48% when threshold is −6 dB, 62% at −8 dB, and 73% at −10 dB;                        For a Reuse Factor of 6:                    In Case 1, can detect 3 or more sites with probability of about 98% when CII threshold is −6 dB; and,            In Case 3, UE can detect 3 or more sites with probability of about 77% when threshold is −6 dB 86% at −8 dB, and 92% at −10 dB.                        
Improving the accuracy in the calculation of the time difference of the serving cell site reference signal and the neighboring cell she will result in an improvement in the accuracy of the location determinations, which will result in enhanced system performance and a reduction of lost data and control signals to and from the user equipment. Increasing the accuracy of the triangulation calculation without requiring extensive system changes or requiring wholesale changes to the reference band or reference signals is needed. Put another way, an improvement in the accuracy of the user equipment positioning analysis when the user equipment is located near the serving cell site is needed, where the improvement attempts to work within the constraints of the existing deployed 3GPP and LTE system and without requiring extensive system changes or new hardware deployment. Based on simulation analysis and comparative studies done on the existing systems and proposals, there is a need to improve the positioning-assisting reference signals so more accurate user equipment positioning can be achieved.
The various components on the system may be called different names depending on the nomenclature used on any particular network configuration or communication system. For instance, “user equipment” encompasses PC's on a cabled network, as well as other types of equipment coupled by wireless connectivity directly to the cellular network as can be experienced by various makes and models of mobile terminals (“cell phones”) having various features and functionality, such as Internet access, e-mail, messaging services, and the like.
Further, the words “receiver” and “transmitter” may be referred to as “access point” (AP), “basestation,” and “user” depending on which direction the communication is being transmitted and received. For example, an access point AP or a basestation (eNodeB or eNB) is the transmitter and a user is the receiver for downlink environments, whereas an access point AP or a basestation (eNodeB or eNB) is the receiver and a user is the transmitter for uplink environments. These terms (such as transmitter or receiver) are not meant to be restrictively defined, but could include various mobile communication units or transmission devices located on the network.